The vast majority of global health problems do not consist so much of finding a cure as delivering one. Improving health in the world's poorest countries requires solutions that are cheap and simple to administer -- and the good news is that these are increasingly available. For example, changing the standard response to diarrhea from saline drips (which require sterile needles and medical staff) to sugar-salt solutions (which require neither) has saved millions of lives that would otherwise have been lost to diseases such as cholera.

Next up may be the scourge of poor sight. There are lots of people who can't read signs, watch TV, or recognize a face across the room without corrective vision. I'm one of the lucky few among them who can afford to do something about it; I have four optometrists within blocks of my office and enough money to buy glasses. But around the world, millions of people who should be able to see clearly are almost blind for lack of corrective treatment. The World Health Organization (WHO) estimates that about 150 million people worldwide who need glasses do not have them. In sub-Saharan Africa, only about 5 percent of people with poor eyesight have glasses. Skilled eye professionals are also extremely rare; Rwanda, a country of 10 million people -- an estimated 1.2 million of whom need eyeglasses -- has just 12 optometrists and ophthalmologists.

On top of eye conditions that can be fixed with glasses, over 20 million people worldwide can't see because of cataracts. Cataract operations are not complex, but they do require a surgeon and a properly equipped hospital. And costs for even a straightforward cataract surgery in the United States range above $3,000 -- or more than 50 times per capita annual health expenditure in Pakistan, for example.

This lack of access exacts a heavy toll on the world's poor. A randomized trial in China suggests that giving glasses to children can have an impact on their performance at school equivalent to an extra half-year in class, which should come as no surprise to any kid who has squinted at the blackboard from the back row of the classroom. Children with poor vision in northeast Brazil are 10 percent more likely to drop out of school and nearly 18 percent more likely to repeat a grade. The WHO estimates the global cost of poor eyesight at $269 billion a year in lost productivity.

Fortunately, innovation in eye-care delivery is reducing the requirements in terms of both financial resources and technical skills needed to correct vision problems worldwide. For example, the Aravind eye hospital network in Southern India has perfected a high-volume, low-cost technique for curing cataract blindness. The approach uses a locally produced replacement lens costing less than $5, which is inserted through a small incision into the eye. A surgeon alternating between two different operating tables can treat 15 cases an hour for less than $15 total (a cost covered for 70 percent of patients by cross-subsidy from the 30 percent of customers who are wealthy enough to pay for it). Some 200,000 cataract surgeries are performed each year by the Aravind network.

Mass-production technologies have slashed the price of glasses as well. China now produces readymade eyeglasses for as little as $2 a pair and made-to-order pairs for $5 to $10 retail. In India, the multinational firm Essilor has funded vans that tour towns and villages offering free eye exams and selling prescription plastic glasses (which cost an average of about $5) and ready-made nonprescription reading glasses (as little as $1), a project that has proved profitable in its pilot stage.

To overcome the shortage of skilled professionals, Kovin Naidoo at the University of KwaZulu-Natal in South Africa is working with community-based health workers to provide simple eye exams, which, combined with cheaper glasses, could considerably increase access to corrective vision. Naidoo's work might be made more straightforward by a new eye-test system developed by the MIT Media Lab, which replaces the traditional eye exam's complex set of corrective lenses and eye chart with a matchbox-size plastic device attached to a cell phone loaded with some simple software. The test takes less than two minutes and doesn't require a skilled practitioner; the snap-on plastic device costs about $1.

Another way of getting around the need for skilled eye-care professionals is "adaptive" eyewear. New lenses filled with silicon oil can be adjusted by the customers themselves to provide corrective vision, and cost about $19 a pair. And a new technology using two lenses which slide across each other to alter focus costs as little as $4, and if production can be scaled up the price could be reduced even further.

It is worth noting, however, that the global vision problem is not simply one of cost and a lack of optometrists. In East Timor, for example, a survey found that 55 percent of rural women would be unwilling to pay even $1 for a pair of glasses. The randomized trial of eyeglasses and education in China found that 30 percent of kids who needed glasses and were offered a free pair refused them; the authors suggest that perhaps their parents were operating under the mistaken view that wearing glasses further damages eyesight. But with marketing approaches that increase the demand for glasses, especially among young people, we can ensure that the whole world can finally see clearly.